If you're designing a cabinet or a bookcase, what's the
greatest length (span) a shelf can be without an objectionable sag? There are
four factors to consider: 1) how the load is distributed, 2) the expected load,
3) the shelf material, 4) the method of reinforcement.

LOAD DISTRIBUTION. For the tests we conducted to create our recommendations
(see below), we wanted to determine the worst possible situation for the distribution
of load. So we use six bricks (42 pounds) and placed them right in the center
of the shelf. However, in a normal situation, the weight would probably be distributed
over the entire shelf.

EXPECTED LOAD. Another factor used to determine maximum span is the total
expected load -- the longer the shelf, the more books (and weight) it has to hold.
A running foot of average sized books weighs about 20 pounds. So a three-foot
shelf filled with average sized books would have to support 60 pounds. Records
albums (does anyone use these anymore) and encyclopedias would weight more, paperback
books less.

SHELF MATERIAL. The third factor used to determine maximum span is the
type of material used -- particle board, plywood, solid wood. Each has a different
stiffness.

REINFORCEMENT. Finally, if you want to increase spans, you can add reinforcement
to reduce the amount of sag.

GUIDELINES. Taking the four factors into consideration, the chart shows
some general guidelines for the maximum span for shelves to avoid objectionable
sag. Note: The most practical approach is to use 4/4 stock or plywood with reinforcement.
This will produce shelves with minimum sag and the best visual appearance.